The conversion of solar energy to electrical current using thin film third generation photovoltaics (PV) is being widely explored for the last two decades. The sandwich/monolithic-type PV devices, consisting of a mesoporous photoanode with an organic/inorganic light harvester, redox electrolyte/solid-state hole conductor, and counter electrode, have gained significant interest due to the ease of their fabrication, the flexibility in the selection of materials and the low cost effective production.
Recently, bulk layers of organometallic halide perovskite based on tin (CsSnX3), or lead (CH3NH3PbX3; X═Cl, Br, I) have been introduced pigment for light harvesting, resulting in high power conversion efficiencies (PCE). These perovskite materials show exceptional characteristics: large panchromatic absorption and very good charge-carrier mobility values being comparable to the amorphous silicon. Minimizing energy losses while favoring charge-extraction rates are fundamental to take advantage of the intrinsic properties of the perovskites and to improve the efficiency.
Therefore, perovskite-based and other types of solid state solar cells generally contain an organic hole transport material (HTM) layer, for transporting holes created by charge separation at the light harvester to the counter electrode and/or cathode for filling up by incoming electrons, thereby closing the electric circuit and rendering the devices regenerative. Currently most performing solid state device use doped Spiro-OMeTAD (2,2′,7,7′-tetrakis(N,N-di-p-methoxyphenyl amine)-9,9-spirobifluorene) as a HTM. The relatively low PCE of solid state devices was often ascribed to the low hole mobility in Spiro-OMeTAD, which causes interfacial recombination losses by two orders of magnitude higher than in electrolyte-based, dye-sensitized solar cells (DSCCs). Further, the use of spiro-MeOTAD as hole transporting material may trigger instability in such solid-state solar cells. Because Spiro-MeOTAD has two oxidation potentials being very close, this HTM in the oxidized form is able to form a di-cation, which in turn can dismutate and might cause device instability. Further, since spiro-OMeTAD compound is present in semi-crystalline form, there is the risk that it will (re)crystallize in the processed form in the solar cell. In addition, the solubility in customary process solvents is relatively low, which leads to a correspondingly low degree of pore filling. Along stability issues, the high cost due to a complicated synthetic route and the high purity that is required (sublimation grade) in order to have good performance have been the main drawbacks for commercial applications of solid state solar cells.
Attempts were made to find an alternate organic HTM having higher charge carrier mobility and matching HOMO level to replace Spiro-OMeTAD. In most of the cases, it is difficult to compete with the performances equivalent to Spiro-OMeTAD-based devices, due to its unique properties: sufficient hole mobility, thermal and UV stability, and well-matched HOMO (highest occupied molecular orbital) energy level to the semiconductor light absorbers.
More recently a number of studies indicated that the HTMs can play a key role in controlling the long-term stability of perovskites solar cell. It was demonstrated that perovskites solar cell are more stable if the organic semiconductors are replaced by carbon as HTMs. Furthermore, it was shown that employing new dopant-free organic HTMs significantly improves the device stability compared to the commonly used chemical doped spiro-OMeTAD.
In view of the above, the present invention addresses the disadvantage of triggering instability and reducing life-time of the device comprising perovskite due to the presence of HTM, e.g. spiro-OMeTAD.
The present invention also pursues to provide new hole transporting material allowing to tune HOMO level and having positive impact on the sensitizer through its passivation to improve and provide higher power conversion efficiency (PCE) to photovoltaic devices comprising perovskite as well as to further optoelectronic devices Organic Light Emitting Diodes (OLED), Field effect Transistors (FET).
The present invention also addresses the disadvantage of the expensive and complex synthesis of HTMs resulting in materials of high price because of the expensive purification steps, starting material compounds, the complexity and the multiplication of reaction steps, the use of aggressive reagents. Thus the synthesis process is lengthy, time-consuming and expensive and causes non-negligible environmental impact.
The invention pursues to provide an efficient solar cell, which can be rapidly prepared in an efficient way, using readily available or low cost materials such as conductive material, using a short manufacturing procedure based on industrially known manufacturing step, keeping the material costs and the material impact on the environment very low.
The present invention addresses the problems depicted above.